Formylphenylboronic acid and its substituted derivatives are customarily synthesized from the corresponding haloaromatic either as described in Liebigs Ann. 1995, 1253-1257 and Chem. Ber. 123 (1990) 1841-1843 in a yield of 94% or 78% by reaction of the corresponding protective bromoaromatic with magnesium in a Grignard reaction and subsequent addition of trialkyl borate or as described in Tetrahedron Lett 1998, 39, 7537-7540, in a yield of 99% by reaction of the corresponding protected bromoaromatic with butyllithium at xe2x88x9278xc2x0 C. and subsequent reaction with triisopropyl borate to form the corresponding formylphenylboronic acid.
However, for the industrial preparation it is disadvantageous that a high purity can be achieved only at very low (xe2x88x9278xc2x0 C.) and thus uneconomical temperatures and with the use of expensive organolithium compounds.
Higher temperatures both in the preparation of the organometallic compound (reflux temperature of THF in the Grignard step) and also temperatures of  greater than xe2x88x9240xc2x0 C. in the addition of the trialkylboric ester onto the organometallic compound frequently result in a product of unsatisfactory purity. The most frequent organic impurities are the corresponding triarylboranes and borinic acids or benzaldehyde and the correspondingly substituted hydroxybenzaldehydes and bisformylbiphenyls which can be formed during the Grignard reaction. 
The impurities interfere when the product is used as precursor for liquid-crystalline compounds, as liquid crystals or as constituent of liquid-crystalline mixtures, in particular when the product is used as pharmaceutical intermediate or generally in applications which require very high purity.
Boronic acids are usually isolated from the reaction mixture by hydrolyzing the suspension from the borate addition and, after phase separation, distilling tetrahydrofuran (THF) from the homogeneous organic phase. During the distillation, the boronic acid precipitates from the solution and can be separated off by filtration.
The organic impurities can be separated off only incompletely, if at all, by this method.
It is possible to remove the above-described impurities by means of multiple extraction with toluene and to recrystallize the boronic acid as described in Chem. Ber. 123 (1990) 1841-1843, with great losses of yield from water or from hydrochloric acid (Synthesis 1999, 2041-2044).
Furthermore, Liebigs Ann. 1995,1253-1257, describes the purification of 4-formylphenylboronic acid by dissolution in aqueous potassium hydroxide at pH 14, extraction of the aqueous solution with methyl t-butyl ether (MTBE) and subsequent precipitation of the boronic acid by means of sulfuric acid. Contamination visible in the NMR of 5-7% is described in the publication. Our own experiments on this showed that, for example, 4-formylphenylboronic acid reacts to a considerable extent in a Cannizzaro reaction in aqueous alkaline medium above a pH of  greater than 11. The 4-carboxyphenylboronic acid and 4-(hydroxymethyl)-phenylboronic acid formed can be separated from the mixture only with great difficulty.
It is an object of the invention to provide a process for purifying formylphenylboronic acid and its derivatives which allows the formylphenylboronic acid to be prepared in high purity and does not have the disadvantages described.
According to the present invention, this object is achieved by a process for purifying formylphenylboronic acids of the formula (I) 
where the formyl function is located in the ortho, meta or para position, preferably in the para position, relative to the boronic acid function and R1 to R4 are each, independently of one another, hydrogen, C1-C12-alkyl, C2-C12-alkenyl, C2-C12-alkynyl, C3-C12-cycloalkyl, C1-C12-alkoxy, O-phenyl, O-benzyl, aryl, heteroaryl, fluorine, N(alkyl)2, N[Si(C1-C4-alkyl)3]2 or CF3, or R1 and R2, and/or R3 and R4, together form an aliphatic or aromatic ring, by dissolving the crude formylphenylboronic acids in an alkaline solvent having a pH in the range from 8 to 11, preferably a pH in the range from 9 to 11, separating off the insoluble organic impurities and subsequently acidifying the alkaline boronic acid solution and separating off and working up the precipitated boronic acid.
In formula (I), it is preferred that R1 to R4 are each hydrogen, C1-C6-alkyl, C2-C6-alkenyl, C1-C4-alkoxy, O-phenyl, O-benzyl, aryl, heteroaryl, fluorine, N(C1-C4-alkyl)2, or CF3, or R1 and R2, and/or R3 and R4, together form a saturated or unsaturated ring having not more than five or six ring atoms.
Particularly preferably, R1 to R4 are each hydrogen, C1-C4-alkyl, C2-C4-alkenyl, C1-C4-alkoxy, aryl, fluorine, N(C1-C4-alkyl)2 or CF3, or R1 and R2, and/or R3 and R4, together form a fused-on cyclohexyl structure, cyclopentyl structure or together with the aromatic ring a naphthyl structure.
In the present process, the isolated crude formylphenylboronic acids are dissolved in an alkaline solvent such as aqueous solutions of alkali metal or alkaline earth metal oxides, hydroxides, carbonates or phosphates. Preference is given to using sodium hydroxide and potassium hydroxide solutions.
In the dissolution, it has to be ensured that the pH is in a range from 8 to 11, preferably from 9 to 11, particularly preferably from 9.5 to 10.5. At pH values of  greater than 11, Cannizzaro products are formed, as described above.
The organic impurities which are insoluble in the aqueous solutions used can subsequently be removed by adsorption on activated carbon or extraction with inert, water-immiscible organic solvents, for example aliphatic hydrocarbons such as various heptanes, octanes, cyclic aliphatic hydrocarbons such as cyclohexane, methylcyclohexane, aromatic hydrocarbons such as toluene, o-, m-, p-xylenes, chlorobenzene, o-, m-, p-dichlorobenzene or ethers such as diethyl ether, diisopropyl ether, dibutyl ether, methyl tert-butyl ether or methyl ethyl ketone or methyl isobutyl ketone, to name only a few.
Preference is given to using toluene, xylene or xylene derivatives, methyl tert-butyl ether.
After the impurities have been separated off, the boronic acid is precipitated again in highly pure form by acidification of the alkaline boronic acid solution. As inorganic mineral acids, preference is given to using sulfuric acid, hydrochloric acid, nitric acid or phosphoric acid. Organic acids which can be used are, for example, formic acid or acetic acid.
Preference is given to using hydrochloric acid or sulfuric acid for acidification. Filtration, washing and drying gives the highly pure arylboronic acid.
The purification, i.e. the alkaline dissolution procedure, is carried out at temperatures of from 5 to 50xc2x0 C., preferably from 5 to 25xc2x0 C., particularly preferably from 5 to 10xc2x0 C. Purification at higher temperatures leads to decomposition products of the formyl function, e.g. in the form of a Cannizzaro reaction to give the corresponding carboxy and hydroxymethyl function. To avoid decomposition of the formylphenylboronic acids, they are thus, according to the invention, dissolved at a pH in the range from 8 to 11. This can be achieved, for example, by suspending the crude boronic acid in water and subsequently adjusting the pH of the solution to a value in the range from 8 to 11 by means of an aqueous base solution or the base itself.
The process of the invention makes it possible to prepare arylboronic acids having a purity of xe2x89xa799%, in particular xe2x89xa799.5%, in a suitable and economical manner. The arylboronic acids obtained in this way are very useful as precursors for liquid-crystalline compounds, as constituents of liquid-crystalline mixtures or as pharmaceutical intermediates.